1. Field of the Invention
The present invention relates to an information recording medium, such as an optical disc or video disc, on which high-density recording can be performed.
2. Discussion of Background
One conventional information recording medium employing an "air-sandwich" construction is disclosed in U.S. Pat. No. 4,074,282 issued to Balas, Jr. et al. This recording medium consists of two discs, each of which may be formed of a transparent plastic plate. Optical information recording layers, which consist of an energy-absorbing layer or a light reflective layer, are formed directly on one surface of each disc. The discs face each other so that the recording layers are positioned therebetween. Each of the discs is bonded with adhesive agent to a concentric inner spacer and a concentric outer spacer disposed therebetween to provide an air space between the discs.
The discs of the above conventional information recording medium must be formed with a relatively small thickness, since the recording/reading beam must pass through the discs without distortion. That is, the use of a disc having small thickness prevents beam energy loss and refraction of light as the beam passes through the disc. However, due to the disc's thickness and transparency requirements, the material used lacks stiffness. Thus, when the air or gas within the space between the discs expands or contracts as the surrounding heat or atmospheric pressure changes, the information recording medium bends or undulates in accordance with this expansion or contraction. Any resulting bending or undulation then causes defocusing of the recording/reading beam, which produces inaccuracies in processing the information stored on the recording layers. In extreme Cases, the information recording medium and the optical head will eventually contact each other thereby preventing any processing of the stored information.
To overcome the prior art disadvantages mentioned above, an improved information recording medium was developed, as disclosed in U.S. Pat. No. 4,564,850 issued to Odawara. This improved information recording medium has a first disc and a second disc coaxially positioned adjacent the first disc to reinforce the first disc. A first spacer is positioned between the first and second discs for fixedly connecting the discs together and for forming an air space between the discs. A recording layer is formed on one surface of the first or second discs within the air space. A second spacer is radially displaced from the first spacer, and positioned between the first and second discs. This second spacer enables the discs to move relative to each other in a radial direction in response to expansion or contraction of at least one of the discs due to changes in temperature, humidity, atmospheric pressure. etc.
Therefore, the information recording medium mentioned above will not bend or undulate due to changes in temperature and atmospheric pressure.
However, in the latter case, other difficulties arise. That is, the information recording medium becomes unduly thick, since the air space is formed between the recording layer and the first or second disc along the whole surface thereof. Further, since the bonding area of the first and second spacers for fixedly connecting with the first and second discs is relatively small, the first and second discs tend to peel from the spacers.